Many examples of the use of surfactants either alone or in combination with other additives to recover residual oil appear in the literature. SPE 84904 reviews some of the various EOR technologies that have been sponsored by the US Department of Energy. SPE 78711 and SPE 100004 describes the use of the Alkali Surfactant Polymer(ASP) process to recover significant residual oil from fields that have been extensively waterflooded. Although these and many more publications and patents support the viability of using surfactants to recover residual oil, some the shortcomings of presently utilized surfactants have not been satisfactorily addressed. A thermally stable, salt tolerant surfactant made from renewable resources that is biodegradable and can be economically manufactured surfactant is desired for EOR.
The alcohol ether sulfonates of the present invention are made from renewable resources and are thermally stable, salt tolerant, biodegradable and can be economically manufactured. They may be used alone or in combination with alkali, polymers, co-solvents, chelating agents and other commonly used oil field additives. The function of the surfactant is to lower the Interfacial Tension (IFT) between the injection brine and the oil trapped in the micoscopic capillary pore spaces within the reservoir and also to change the wettability of the reservoir rock. The function of the polymer, that is employed with the surfactant in some cases, is to increase the viscosity of the injection brine in order to overcome viscous fingering and to improve the sweep efficiency of the injected brine. Alkali is also used in one cases to reduce the amount of surfactant and/or polymer adsorbed onto the surface of the reservoir rock and also to react with the small amount of acids and acid precursors present in most crude oils thus forming “in situ” surfactant that acts synergistically with the injected surfactant to give even lower IFT.
Ether sulfonates are a class of surfactants that have been found to be both salt tolerant and thermally stable. Currently available ether sulfonates are limited in their use in EOR because they are either very costly to manufacture or they involve the use of very toxic intermediates for their manufacture. For example U.S. Pat. No. 5,075,042 to Allison et al. describes the preparation of aliphatic poly(ethleneoxy)sulfonates by the chlorinatiuonal with thionyl chloride of an ethoxylated aliphatic alcohol and subsequent conversion of the resulting chloride to the sulfonate with sodium sulfite.
U.S. Pat. No. 4,865,774 to Fabry, et al. in 1989 discloses the synthesis of surface-active hydroxsulfonates made through the reaction of unsaturated fatty alkenyl or fatty alkenylpolyalkoxy esters. This patent describes the procedure for preparing such surface-active agents, for example, from oleyl alcohol. The alcohol is first esterified with acetic anhydride. The acetate ester is used to produce a final product containing sodium acetate as a by-product. It is not necessary to start with the acetate ester if the final product is to be used in soft water since the sodium sulfate formed when the final product is neutralized will not cause any precipitation problems in the absence of significant concentrations of divalent cations. This is followed by sulfonation with SO3 and neutralization with sodium hydroxide (NaOH). The procedure was also carried out starting with oleyl alcohol with 2 moles and 5 moles of ethylene oxide (EO). Calcium hydroxide and ammonium hydroxide were also used to neutralize the acid formed during sulfonation and to remove the acetate. The final products were tested and found to be an excellent wetting agents suitable for inclusion as components in cleaning compositions. We have now found that these type of surfactants are excellent IFT reducing compounds suitable for inclusion in injection brines for application at high temperatures and/or in high TDS brines even those containing significant amounts of divalent cations such as calcium and magnesium. This is unexpected considering the structure of these surfactants is such that the oil soluble hydrophobic and the water soluble hydrophilic portion of the molecule are not distinctly separated.